Unit 2: Anatomy of the Cerebrovascular System

This unit includes a review of arterial anatomy relevant to our discussion of cerebrovascular disease. You may access an enlarged version of the radiographic images where indicated for better viewing. Click on the links below to view each topic area. This page has several images, so please be patient as they are downloaded.

The right and left common carotid arteries bifurcate into the external carotid and internal carotid arteries at about the level of the fourth cervical vertebra (C4). The external carotid arteries and their branches supply the neck, facial structures, and scalp. Branches of the external carotid include the facial, lingual, and superior thyroid arteries anteriorly, and the ascending pharyngeal, occipital, posterior auricular, superficial temporal, and internal maxillary arteries posteriorly.¹

Each ICA enters the cranium through the carotid canal at the base of the skull. It takes an S-shaped course beside the sella turcica, an area referred to as the carotid siphon, and ends in bifurcation laterally and posteriorly to the anterior clinoid process.¹ The branches of the ICA supply the anterior, middle and lateral portions of each cerebral hemisphere. The major cerebral branches of the internal carotid artery are the anterior cerebral and middle cerebral arteries, and the posterior communicating (P-com) artery which joins the ICA to the posterior cerebral artery, forming a segment of the Circle of Willis. Notice the cross-filling of contrast from the anterior circulation across the P-com into the PCA in Fig. 2.2

Each anterior cerebral artery (ACA) courses medially to form the anterior segments of the Circle of Willis (Fig. 2.3, 2.6), then courses superiorly, branching to supply the frontal lobe, basal ganglia, and portions of the parietal, and temporal lobes including the lateral ventricles and corpus callosum.² The right and left ACA's are joined across the midline by the short anterior communicating (A-com) artery to complete the anterior portion of the Circle of Willis. Notice in Fig. 2.3 the cross-filling of contrast from the left ACA across the midline via the A-com into the right ACA circulation. Radiographically, the branches of the ACA delineate the midline as they outline the medial surfaces of the frontal and parietal lobes, within the longitudinal fissure. Displacement of these vessels from their normal position as seen on a frontal angiographic image is referred to as "midline shift", and indicates the presence of a nearby mass, such as an aneurysm, edema, hematoma, or tumor. As the mass encroaches upon and displaces healthy tissue, injury can result. This is referred to as the "mass effect". Embolism involving the anterior cerebral is relatively uncommon.³

The middle cerebral artery (MCA) is the largest branch of the internal carotid artery, and serves large portions of the frontal, parietal, and temporal lobes, including the basal ganglia, motor, sensory and auditory areas, and the lateral surface of each cerebral hemisphere.^{1, 2} It extends horizontally and laterally from its origin before branching at the Sylvian fissure. The subsequent ascending branches outline the insula, a triangular shaped area of the cerebrum at the floor of the Sylvian fissure. The appearance of these vessels in this area, referred to as the Sylvian triangle, is diagnostically important, and can be best observed on the lateral projection of an internal carotid arteriogram. The MCA is the most commonly affected vessel in embolic stroke.³

Because the two vertebral arteries join before continuing into the posterior circulation, catheterization of both during arteriography is not usually necessary. Because the LVA is usually slightly larger in calibre than the right and arises at a less acute angle from its origin, it is usually selected for catheterization. The contrast will often reflux into the opposite vertebral artery during injection, thereby demonstrating both vessels with a single contrast injection. Notice the bilateral filling of both vertebral arteries demonstrated in Fig. 2.5.

The Circle of Willis is the most common location for congenital "berry" aneurysms. Aneurysms of the anterior communicating artery constitute 34% of all intracranial aneurysms, while those of the first segment of the anterior cerebral artery account for 28%.⁴